In order to meet some emission regulations, selective catalytic reduction systems using externally added reducing agents can be used. In such a system, regulated emissions, such as certain nitrogen oxides, or NOx, can be reduced in a oxygen-rich environment to nitrogen and water over a catalyst when a reducing agent, such as ammonia, is added. In addition to controlling nitrogen oxide emissions, the amount of excess ammonia, or ammonia slip, must be managed. Ammonia slip occurs when ammonia in excess of that used to reduce the nitrogen oxides passes through the catalyst unaffected and exits the catalyst (as ammonia slip).
One method for regulating nitrogen oxide emissions and ammonia slip is to use an after-catalyst NOx sensor to detect nitrogen oxide concentration. Control of NOx emissions are allegedly achieved by varying reductant injection until the level or quantity of nitrogen oxides as measured by the sensor falls within an acceptable limit. The amount of reductant injected to keep NOx emissions within the acceptable limit needs to be balanced with an ammonia slip limit. This can be measured and controlled by an after-catalyst ammonia sensor. Such a system is disclosed in U.S. Pat. No. 5,233,934. Alternatively, ammonia slip can be calculated and controlled using an algorithm. Such a system is disclosed in U.S. Pat. No. 4,751,054.
The inventors herein have recognized a disadvantage with the above systems. The above systems attempt to control nitrogen oxide emission level, while limiting ammonia slip. However, these systems do not consider NOx conversion efficiency. While NOx conversion efficiency and after-catalyst NOx emission levels are related, there is an important distinction in their use for reductant control strategy. In general, as maximum NOx conversion is approached with increasing ammonia addition (i.e., increasing NH.sub.3 /NOx mole ratio), ammonia starts to slip. After maximum NOx conversion is attained, ammonia slip increases more rapidly with increasing NH.sub.3 /NOx. For example, if a NOx emission level is regulated to a specific concentration value, then at high feed gas NOx levels, the demand for NOx reduction can easily result in attaining a NOx conversion where ammonia slip is likely excessive and prone to go out of control.
In other words, because a catalyst experiences widely varying levels of engine NOx, controlling to a specific concentration value results in widely varying, and less than optimum, NOx conversion efficiency. Thus, prior art methods are insufficient.